1. Technical Field
The present invention relates to a developing roller manufacturing method, a developing roller, a developing device, and an image forming apparatus.
2. Related Art
In an image forming apparatus using nonmagnetic mono-component toner, an electric charge is given to toner on a developing roller by frictional electrification. In order to effectively generate the frictional electrification, in JP-A-2001-66876, there is disclosed a developing roller in which blast processing is carried out on the developing roller, so that a given surface roughness Rz is given to the surface of the developing roller, whereby toner can be effectively rubbed against the developing roller.
However, the recess portions formed by the blast processing are not uniform in size, depth, shape, or array. For this reason, there is a probability that toner entering a deep recess portion cannot be rolled, so that it cannot be effectively electrically charged. In this manner, there is a probability that filming will occur due to the unevenness of the concave-convex portions of the developing roller surface. Further, in the case where toner is not effectively electrically charged, there is also a problem that the toner leaks from the developing device, thereby being dispersed in an image forming apparatus, or there may be ground fogging of an image.
Therefore, in order to improve the electrical charging of toner, in JP-A-2007-121947, there is disclosed a developing roller in which grooves regularly arrayed in a lattice shape on the developing roller are formed by rolling working, and a manufacturing method thereof. It has been reported that the developing roller with such grooves has improved electrical charging compared to conventional developing rollers having an irregular surface state due to blast processing.
However, also in the developing roller disclosed in JP-A-2007-121947, there is a thinly colored filming. The filming grows in accordance with the repeat of printing, so that the surface of the developing roller appears to be gradually colored by the color of the toner. Therefore, it will be hereinafter referred to as “colored filming”. If the extent of the colored filming becomes excessive, adequate electrical charging cannot be applied to the toner, and therefore the amount of scattered toner is increased, so that there will be ground fogging of an image.
The cause of the colored filming can be considered as follows. There are regions in which toner cannot sufficiently circulate on the inclined surfaces and the like of a convex portion surrounded by the lattice grooves formed by the rolling working, so that toner is retained in the regions. When the temperature of the developing roller surface is increased due to consecutive printing or the like, the retained toner is affected by the heat, so that the filming progresses gradually. In a case where the grooves are formed by the rolling working, there are many cases where a protrusion (hereinafter referred to as a “burr”) is formed on the ridge line of the convex portion surrounded by the grooves. The toner is apt to be retained in the portions which are in the shade of the burr, and this causing the generation of the filming.
The toner remaining on the developing roller without being developed is usually removed by a supply roller and new toner is supplied. Consequently, the same toner does not continually stay at the same place. However, if a burr exists on the ridge line of the convex portion, as shown in FIGS. 12A and 12B, regardless of the rotation direction of the supply roller, the old toner cannot be completely removed. For this reason, toner retention occurs in the proximity of the burr. As a result, the retained toner develops into the filming.